The invention relates to a method and to a device for operating a turbomachine comprising an inlet and an outlet for a process gas so as to limit the soiling of the internal parts of the turbomachine with dirt originating from the process gas.
The soiling of the internal parts of turbomachines, particularly centrifugal compressors, is a phenomenon that the user finds difficult to control or prevent.
The deposition and build-up of dirt on the aerodynamic internal parts of turbomachines may have consequences which are completely undesirable as far as performance is concerned.
On the one hand, the process carried out in the turbomachine may be modified substantially. In the case of a centrifugal compressor, the pressure and temperature levels or the rates of flow through the compressor may be modified because of the formation of deposits in the aerodynamic passages such as the vanes or the diffusers of the compressor.
On the other hand, the mechanical parts of the turbomachine may be subjected to stresses which cause them to deteriorate. It is therefore necessary to protect these mechanical parts. In particular, imbalance, variations in axial thrust, or soiling of the internal packing brought about by deposits on the dynamic parts of the turbomachine may give rise to vibrations which are detrimental to the correct running of the turbomachine.
The deposition and build-up of dirt on the internal parts of the turbomachines and, in particular, centrifugal compressors, are due to two main causes. First of all, the filters or separators placed upstream of the turbomachines are unable to hold back particles with a particle size of a few micrometers, which become deposited on the internal parts of the turbomachine. Furthermore, the pressure and temperature levels reached in the compressor, and the nature of the gases being compressed, encourage reactions of the polymerization type in the substances deposited or encourage the internal parts of the compressor to corrode under the effect of the substances deposited.
In general, the soiling of the internal parts of turbomachines and, in particular, of centrifugal compressors, is a general phenomenon which occurs in all cases during normal operation of the turbomachine. This soiling may reach such a level that it becomes necessary to shut down the turbomachine and therefore the production or manufacturing cycle that is in progress. It is therefore entirely desirable to have available means that allow dirt to be removed from the soiled internal part of a turbomachine or that allow the deposit of dirt in this internal part to be limited.
Hitherto, no general method allowing the internal parts of turbomachines to be cleaned, regardless of the type of turbomachine concerned, of the substance circulating through these turbomachines and of the type and nature of the dirt likely to be deposited in their internal parts, is known.
Each turbomachine operator attempts to remedy the soiling problem he encounters according to the type of soiling or according to the production organization characteristics.
Methods involving anti-soiling coating or solvents or chemical additives that make it possible to reduce or eliminate soiling in certain specific cases are known. In general, with a view to optimizing the availability of industrial equipment, the main methods used and which can be combined with one another, consist in:
removing and sand-blasting the soiled parts of the turbomachines,
periodically injecting solid or liquid particles (particularly in the form of a mist) in order to erode or dissolve the dirt,
constantly mixing additives into the fluid passing through the turbomachine, these substances preventing or slowing polymerization,
coating the internal parts with coating to produce non-stick surfaces.
All these methods exhibit drawbacks. In particular, these methods are expensive and their effectiveness is neither complete nor lasting.
Furthermore, each of these methods is tailored to a specific case and no method capable of widespread application is known.
Cleaning methods which are applied outside the sector of operation of the turbomachines and which use a solvent consisting of a dense fluid under pressure such as carbon dioxide, in the liquid state or alternatively in the supercritical state, are also known.
In such methods, the carbon dioxide may be used in place of organic solvents.
Carbon dioxide CO2 has a critical point at a pressure of 73 bar (7.3 MPa) and a temperature of 31xc2x0 C.
These cleaning methods employ carbon dioxide at a pressure higher than the critical pressure and at a temperature which may be lower than the critical temperature, the carbon dioxide then being liquid, or alternatively at a temperature higher than the critical temperature, the carbon dioxide then being in a supercritical state that is intermediate between the liquid and gaseous states.
The critical pressure and temperature values for CO2, which are not very difficult to achieve, allow industrial application.
In the supercritical state, the properties of CO2, such as its density, its viscosity which is slow, and its diffusion coefficient which is high, and its very good dissolving power with respect to numerous substances, make it a solvent product that is advantageous for the purposes of cleaning, purifying and treating materials.
In the supercritical state, CO2 in particular dissolves most organic compounds.
Other substances, such as certain alkanes, may have similar properties in the supercritical state.
In the case of turbocompressors which have an inlet into which a gas involved in a process in which the gas undergoes a physical or chemical transformation is introduced, it is generally desirable for the dirt inside the turbocompressor to be removed continuously while this turbocompressor is in operation. It has been proposed that a substance capable of dissolving the dirt deposited inside the turbocompressor be introduced into the stream of process gas at the inlet of the turbocompressor.
At the outlet of the turbocompressor, fluid consisting of the process gas and of the substance in the supercritical state containing the dirt in the dissolved state is collected. The process gas and the fluid consisting of the substance containing the dirt in the dissolved state have then to be separated.
In order to clean the compressor under economical conditions, it is obviously desirable for the substance used to dissolve the dirt in the internal parts of the turbocompressor to be regenerated and recycled. In order to do this, it is necessary to separate the impurities consisting of the dirt which has been dissolved by the substance in the supercritical state to be separated from the substance used for cleaning. This separating of the impurities cannot be achieved continuously, in the stream of dissolving substance circulating through the compressor under conditions which are economical enough to be acceptable in the context of an industrial process.
The reason for this is that in order to separate impurities continuously in the stream of dissolving substance, it is generally necessary to get around the critical point of the fluid by thermodynamic transformations in a well-defined order. It is necessary to reduce the pressure of the substance in order to cause it to vaporize, the impurities in the liquid or solid state thus being separated from the substance in the gaseous state.
It is then necessary to recompress the substance in order to reintroduce it into the process circuit, inside the compressor, in a supercritical state. In order to pressurize the dissolving substance it is necessary to use a high-delivery compressor or pump, the installation and running costs of which are generally incompatible with economical implementation of an industrial process using the process gas.
It is therefore desirable to have available a method for operating turbomachines which makes it possible to limit the extent to which they become soiled, without having to regenerate and recycle a cleaning substance continuously while the turbomachine is operating.
The use of the compressor to circulate the dissolving substance is generally incompatible with the rating of the compressor, because of the pressure and developed power level needed for a constant rotational speed
The object of the invention is therefore to propose a method for operating a turbomachine comprising an inlet and an outlet for a process gas circulated through a circuit known as the process circuit, that makes it possible to limit the soiling of the internal parts of the turbomachine with dirt originating from the process gas, without having to constantly circulate, regenerate and recycle a cleaning substance throughout all the operating phases of the turbomachine.
To this end, the method according to the invention is characterized in that, in between at least two successive phases in which the turbomachine is operated normally, during which phases only process gas is introduced into the inlet of the compressor and the process gas is recovered for use, a cleaning phase is carried out during which a substance in a dense state capable of dissolving the dirt on the internal parts of the turbomachine is introduced into the process circuit at the inlet of the turbomachine and the process gas and the substance in which the dirt is dissolved in the form of impurities in the liquid state are separated.
In order to provide a clear understanding of the invention, a method for operating a turbocompressor making it possible to limit the soiling of the turbocompressor and the device used to implement the method will be described by way of example with reference to the appended figures.